This invention relates to a clutch of the friction type in a drive line of a motor vehicle, and more particularly to a system for automatically actuating the clutch including a control apparatus for controlling a clutch actuating fluid pressure with compensation for a decrease in the cofficient of friction of the friction surfaces caused by an increase in the clutch temperature.
With regard to a conventional clutch of the friction type in a transmission line of a motor vehicle, it has been proposed to employ an actuating system which automatically applies a controlled clutch engagement force to a pressure plate of the clutch at starting of the vehicle by means of a fluid pressure in a programmed relation to the position or opening degree of a throttle valve for the engine of the vehicle. A problem in such a clutch actuating system is the influence on smooth engagement of the clutch of an inevitable decrease in the cofficient of friction of the friction surfaces with increase in the temperature of the clutch disc. Since the rate of variation in the clutch actuating pressure is programmed on the basis of a standard clutch temperature, a satisfactorily smooth engagement of the clutch and hence a smooth starting of the vehicle cannot be realized if the clutch temperature, particularly the clutch disc temperature, is excessively high due to either an immoderate use of the clutch or use in summer.
It has been tried, therefore, to estimate a decrease in the coefficient of friction by detecting an increase in the temperature of the clutch disc and regulate the clutch actuating pressure to compensate for the decrease in the coefficient of friction. However, it is not easy to detect the temperature of a rotating clutch disc in a motor vehicle. Ordinary devices for use in detecting the temperature of a rotating article such as slip rings, thermistors and transistors are not fully suitable to the application to a vehicular clutch due to inadequacy in their various properties such as shape, durability, shock and vibration resistance and weather resistance. In fact, no practicable means for detecting the temperature of a vehicular clutch disc is available.